Atrait that confers a greater level of fitness, relative to those who lack it, is called a(n)

Adaptation

Explanation:

The adaptation is the process by which a species becomes fitted to its environment which is a direct result of natural selection.

The traits that best suits the organism in an extreme conditions and helps in better survival is known as adaptation.

The variation in the species can be favored or rejected based on the condition. If the variation helps in survival of the organism in a better way then it is favored and if it is does not favors its survival then it is not carried further.

The best suited character is adapted and carried forward.

sheep is a herbivore animal whereas grasshopper belongs to the class chordates but both of them feed on greens.  

if grasshopper and sheep feeding on a pasture land, i do not think both of them will have any impact on each other. however, the number of the species can make difference. since sheep feed on greens, there is a possibility there left no pasture land for the grasshoppers or they may get extinct.